IF YOU HAVE CHILDREN, and even if you don't, take a few mins to read the Free To Play Monetization Tricks article.

Quite an eyeopener.

EDIT: From the author in the comment section:

Nothing in these games happens by accident. Companies like King have so many data analysts (probably on par with actual designers) that the actual game mechanics becomes secondary to trying to optimize monetization.

EDIT2: I've read most of the comments at this point. Damn, this rabbit hole goes deep. Fascinating stuff.